The present invention generally relates to implantable medical devices, monitoring systems and associated procedures. More particularly, this invention relates to a sensor unit comprising an anchor and an implantable medical sensing device, and to a procedure for implanting the sensing device for monitoring intracranial physiological properties.
Wireless devices such as pressure sensors have been implanted and used to monitor heart, brain, bladder and ocular function. With this technology, capacitive pressure sensors are often used, by which changes in pressure cause a corresponding change in the capacitance of an implanted capacitor (tuning capacitor). The change in capacitance can be sensed, for example, by sensing a change in the resonant frequency of a tank or other circuit coupled to the implanted capacitor.
Telemetric implantable sensors that have been proposed include batteryless pressure sensors developed by CardioMEMS, Inc., Remon Medical, and the assignee of the present invention, Integrated Sensing Systems, Inc. (ISSYS). For example, see commonly-assigned U.S. Pat. Nos. 6,926,670 and 6,968,734 to Rich et al., and N. Najafi and A. Ludomirsky, “Initial Animal Studies of a Wireless, Batteryless, MEMS Implant for Cardiovascular Applications,” Biomedical Microdevices, 6:1, p. 61-65 (2004). With such technologies, pressure changes are typically sensed with an implant equipped with a mechanical (tuning) capacitor having a fixed electrode and a moving electrode, for example, on a diaphragm that deflects in response to pressure changes. The implant is further equipped with an inductor in the form of a fixed coil that serves as an antenna for the implant, such that the implant is able to receive a radio frequency (RF) signal transmitted from outside the patient to power the circuit, and also transmit the resonant frequency as an output of the circuit that can be sensed by a reader outside the patient. The implant can be placed with a catheter, for example, directly within the heart chamber whose pressure is to be monitored, or in an intermediary structure, for example, the atrial or ventricular septum of the heart.
Presently in the United States, roughly one million people are treated for head injuries each year, with over a quarter million of these being moderate or severe injuries. Traumatic brain injuries currently account for approximately 70,000 deaths each year in the United States, with an additional 80,000 patients having severe long-term disabilities. Monitoring intracranial pressure (ICP) to identify intracranial hypertension (ICH) is one of the most important steps in treatment of severe head injuries. The ability to accurately monitor and identify high ICP levels enables physicians to diagnose and treat the underlying causes and significantly reduce the morbidity and mortality rates of these patients.
ICP is currently measured and recorded through a variety of systems, such as intraventricular catheters, subarachnoid bolts, and catheter tip strain gauges. However, each of these systems has significant drawbacks, including the need for repositioning and balancing, the occurrence of occlusions and blockages, and the risk of infection.